Practical Pulsed Dipolar ESR (DEER)

Abstract

The recent resurgence of ESR (electron spin resonance) in structural biology is in large part due to the development of distance measurements. This application was made possible by targeting of specific sites/domains by cysteine mutagenesis. Four techniques were developed to cover various distance ranges: exchange ESR for the very short distances (4–8 Å) (Miick et al. 1992); static and dynamic dipolar cw ESR (continuous wave ESR) for the 8–25 Å distance range (McHaourab et al. 1997; Rabenstein and Shin 1995); and two pulsed methods: DEER (double electron electron resonance) (Milov et al. 1981) and DQC (double quantum coherence) (Borbat et al. 2002) for distances between 17 and 80 Å. The first two methods rely on line shape broadening and are thus limited to strong interactions (short distances). The pulsed methods extract weaker dipolar interactions from spin coherence and are thus sensitive to longer interspin distances. In addition, distances can be measured between the nitroxides and paramagnetic metals that enhance the relaxation of nitroxides (Budker et al. 1995; Voss et al. 1995). All these techniques have been extensively tested and verified across the full distance range of sensitivity on model systems including organic biradicals (Jeschke et al. 2000), proteins (Sale et al. 2005) and DNA (Borbat et al. 2004; Schiemann et al. 2004). The use of extrinsic spin labels for distance measurements allows targeting of specific sites by side-directed spin labeling, which is of great advantage.